1. Field of the Invention
The invention relates to a projection display, more particularly to a triple-lens type projection display with uniform optical path lengths for different color components.
2. Description of the Related Art
Referring to FIG. 1, a conventional triple-lens type projection display 1 is shown to comprise a light source 11, a light filter 12 downstream of the light source 11 for removing ultraviolet rays and infrared rays from a light beam generated by the light source 11 to thereby obtain a white primary beam output, a first expansive lens 141 downstream of the light filter 12, a first beam splitter 131 downstream of the first expansive lens 141 to split the primary beam output from the first expansive lens 141 into a first color component and a secondary beam output, a second expansive lens 142 downstream of the first beam splitter 131, a second beam splitter 132 downstream of the second expansive lens 142 to split the secondary beam output from the second expansive lens 142 into second and third color components, and a third expansive lens 143 downstream of the second beam splitter 132. The first, second and third color components are generally primary color components, such as red, green and blue. The first color component from the first beam splitter 131 passes sequentially through a first focusing lens 151, a first polarizer 161, and a light-modulated first light valve 171 before being received by a first projection lens 181. The second color component from the second beam splitter 132 passes sequentially through a second focusing lens 152, a second polarizer 162, and a light-modulated second light valve 172 before being received by a second projection lens 182. The third color component from the third expansive lens 143 is reflected by a mirror 133 so as to pass sequentially through a third focusing lens 153, a third polarizer 163, and a light-modulated third light valve 173 before being received by a third projection lens 183. The first, second and third projection lenses 181, 182, 183 are disposed on a common plane, and project light onto a display screen 19 for showing an image on the latter.
It is noted that, while the distances of the first, second and third projection lenses 181, 182, 183 from the respective one of the first and second beam splitters 131, 132 and the mirror 133 are equal, the distance of the second beam splitter 132 from the light source 11 is longer than that of the first beam splitter 131, and that the distance of the mirror 133 from the light source is longer than that of the second beam splitter 132. There is thus a need to install the expansive lenses 141, 142, 143 to compensate for the differences in the optical path lengths traveled by the different color components.
However, because three expansive lenses 141, 142, 143 are needed for the three focusing lenses 151, 152, 153, the conventional projection display 1 involves a relatively large number of essential components. In addition, the expansive lenses 141, 142, 143 can introduce edge distortion to the image 191 shown on the plane of the light valves, as shown in FIG. 2.
Therefore, the main object of the present invention is to provide a triple-lens type projection display with uniform optical path lengths for different color components, thereby eliminating the need for expansive lenses as required in the aforesaid prior art.
According to this invention, a triple-lens type projection display comprises:
a source for generating a white primary beam output;
a first beam splitter for splitting the white primary beam output into a first color component and a secondary beam output;
a second beam splitter for splitting the secondary beam output into a second color component and a third color component;
first, second and third light modulators for modulating the first, second and third color components, respectively;
a first projection lens for receiving the first color component from the first light modulator;
a second projection lens for receiving the second color component from the second light modulator; and
a third projection lens for receiving the third color component from the third light modulator.
Each of the first, second and third color components has a respective optical path length that is measured from the first beam splitter to a respective one of the first, second and third light modulators. The optical path lengths of the first, second and third color components are equal.